Due to concerns over fuel use, it has become imperative for automotive manufacturers to adopt an ecologic mindset. In this mindset efficiency plays a predominant role when developing new vehicle systems. A new car must waste as little fuel as is practical, hence increasing the mileage that the car achieves, and so reducing the cost of running the vehicle and its environmental impact.
One particular feature that can be introduced to minimize the use of fossil fuels is a stop start system. A stop start system automatically shuts down the engine in a car when it is not in use, and starts it again as soon as it is required. The shutdown in a stop start system is not a full shutdown, but a partial shutdown which allows the engine to restart promptly when needed. By using a stop start system when a car is idling, for example at traffic lights, fuel consumption can be reduced. Similar systems are frequently used in hybrid vehicles, where they can be used even if the car is not stationary. For example, the internal combustion engine may not be employed during low power uses of the vehicle and while the high voltage battery system has sufficient power to drive the vehicle. A stop start system can also be used in purely internal combustion engine driven vehicles to conserve fuel during stationary idling.
One major problem which must be overcome in a stop start vehicle is how to imperceptibly re-crank the engine, starting the engine again when it is required. Re-cranking the engine requires considerable power from the primary battery in the car, typically a 12 volt battery, and as such re-cranking can cause a drop in the voltage available to other systems. This can cause control modules to reset and other faults to arise. If a vehicle is completely shut down before start up, (e.g. when an ignition system is disabled) these problems do not arise since any fault codes which arise due to a voltage drop during the initial crank are not logged. These fault codes can be safely ignored since the initial crank is not a safety critical scenario. However once the vehicle has been driven, and during a re-crank, similar errors cannot be ignored. Therefore many systems in the vehicle require a reliable voltage to continue functioning, and to prevent inaccurate faults being logged. These systems may include control components, and in particular engine management systems require a reliable voltage and will generally go into shut down when a predetermined voltage reduction has occurred. Since several of the electrical systems in a car are safety critical, it is vital that a reliable voltage is provided. Safety critical systems include some braking systems, stability control etc. If the car is a hybrid it may still move while the engine is turned off, and steering systems may therefore be safety critical as well.
It is also important to perform a re-crank without an audible or visible drawback for the driver, such as the lights dimming, or the volume of a radio dropping, or any similar effect.
To tackle this issue, a new hardware layout has been designed in which two batteries, typically 12V each, are connected in parallel. International patent application PCT/EP2012/051484 describes one such system. In a two battery system, the auxiliary battery can be used to support control modules and other systems while the main battery is used to re-crank the engine. This requires a control system that isolates the main battery from the vehicle systems during re-crank, otherwise the effect of two batteries may be lost.
In a conventional vehicle with a stop start system, the auxiliary battery is only used briefly, during a re-crank. It is desirable that the auxiliary battery is not cycled significantly, so that it lasts a long time before requiring replacement.
In a hybrid vehicle, in contrast, it is typically desirable to isolate the main battery from the remainder of the electrical system for as long as the internal combustion engine is turned off. Hence the 12V auxiliary battery is used whenever the hybrid is in an electric mode, relying upon a high voltage battery driven motor rather than an internal combustion engine. Hybrid vehicles often have at least three batteries, two of which are 12V batteries and one of which is a 300V battery.
It is suggested in PCT/EP2012/051484 to isolate the auxiliary battery during normal operation to avoid this cycling problem, and this is achieved by a switch which isolates the auxiliary battery from the other electrical components of the vehicle. However, it is necessary to keep the auxiliary battery at full charge to avoid deterioration, and to ensure that the battery is ready for use by the stop start system.
Typically, the battery in a conventional vehicle is monitored for its condition via signals obtained from a Battery Monitoring System (BMS), which measures the battery temperature, battery voltage and battery current. With this information, the BMS derives the battery's State Of Charge (SOC), so that the battery can be charged as necessary.
For a car with two batteries, the attributes of the auxiliary battery also require measurement if that battery is to be used properly. In particular, the performance of the auxiliary battery (and in turn the performance of any system which relies upon the auxiliary battery during a re-crank) depends upon the auxiliary battery's SOC. For this reason another BMS module, attached to the auxiliary battery, can be utilised to determine the SOC. However, BMS modules are expensive, and fitting an additional one increases the cost of manufacture. Therefore, if the estimation of the auxiliary battery SOC could be realised without the use of a second BMS module, this would result in a cost saving.